1. Field of the Invention
The present invention relates to an optical communications system, and more particularly to an optical communications system employing wavelength-division multiplex techniques.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) techniques are widely used in today's optical communications infrastructures. The WDM technology enables many transmission signals to be carried over a single fiber-optic medium, assigning different optical wavelengths to different channels. SONET/SDH data streams, for example, are transported on a WDM network, where optical signals sent out from a multiplicity of transmission units are multiplexed (combined) into one output optical fiber for delivery to peer network equipment. At the receiving end, the signals are demultiplexed (split) into the original data streams and distributed to their destinations.
Recent WDM networks can actually convey as many optical signals as more than one hundred channels in multiplexed form, and it would not be efficient to convert them back to electrical signals for the purpose of setting and changing their delivery paths. For this reason, there has been a demand for all-optical path switching facilities using optical switches which is able to configure signal routes on an individual wavelength basis.
Researchers have developed optical switch modules as the core technology of all-optical cross connect systems. More specifically, some systems perform wavelength routing with micro-electro-mechanical systems (MEMS) mirror arrays that are combined in an appropriate way. Other systems realize path switching by changing the temperature of optical waveguides. Another solution is the system disclosed in the Unexamined Japanese Patent Publication No. 6-35012 (1994), which employs wavelength converters to alter the wavelengths of received optical signals as required. It then directs the converted signals to optical multiplexers and demultiplexers, thereby realizing optical switching.
While such optical switch modules play an essential role in conventional WDM networks, they are expensive and still immature. Development of large-scale matrix switches has been underway, but not yet in the actual use. It is therefore desired to develop an optical communications system which offers flexible and efficient optical cross-connect functions by using ordinary components instead of costly optical switch modules.